In the construction of a metal buiding, it is common to use a frame which supports parallel spaced steel beams or purlins to which corrugated sheet metal roof panels are attached. The purlins have a Z-shaped cross-sectional configuration, for example, as illustrated in U.S. Pat. No. 2,871,997, No. 3,290,845, No. 3,982,373 and No. 3,513,614. Similar Z-shaped beams are attached to the sides of the frames and are commonly referred to as wall girts for supporting the roll-formed sheet metal side wall panels. The most commonly used purlins are roll-formed from a relatively heavy gauge steel strip, such as fourteen gauge (0.074"), and have a height of approximately eight inches. Thus the flange portions of a purlin are integrally connected by the flat web portion, and all of the portions have a common uniform thickness which results in a weight of approximately 3.67 pounds per linear foot for an eight inch purlin.
While eight inch purlins are most commonly used because of its strength/weight ratio, purlins having a greater height, such as twelve inches, are also used in view of the fact that the strength of the purlin increases as the square of the height. However, as the flat web portion of a conventional purlin increases to provide the purlin with a greater height, the additional thickness of steel needed in the web portion adds little to the increased strength but substantially increases the weight of the purlin.
It has also been found desirable for the flange portions of a purlin to have a substantially flat outer surfaces to provide proper attachment of the purlins to the metal frame and of the roof panels to the purlins by suitable fasteners. While a one-piece conventional Z-shaped purlin may be efficiently manufactured by roll-forming, it does not provide maximum utilization of the strength of the steel and thus does not obtain the maximum strength/weight ratio. While there have been many other types of sheet metal beams either proposed or made, none of these beams have been found satisfactory for replacing the above described purlins commonly used in the construction of a metal building.